| Internet-Draft | abandon-upa-proposal | May 2025 |
| Wang | Expires 17 November 2025 | [Page] |
[I-D.ietf-lsr-igp-ureach-prefix-announce] (UPA draft) proposes the solution to announce the prefix unreachable information within the IGP domain.¶
It utilizes the LSInfinity concept that is introduced in [RFC2328], without analysis the dormant and flawed design.¶
It defines some new flags to convey the maintenance information, which is not the role of IGP protocols and finally, there are several parts that are overlap with another precedent draft that discusses the corresponding contents more throughly.¶
This document analyzes the above issues, suggests the IETF commuity abandon the UPA draft, replace it with other more comprehensive document [I-D.wang-lsr-prefix-unreachable-annoucement](Founder Draft), to provide the IETF community the more optimal solution.¶
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[I-D.ietf-lsr-igp-ureach-prefix-announce] (UPA draft) describes one proposal that tries to announce the prefix unreachable information within the IGP domain.¶
It explores one seldom used LSInfinity feature that is defined in [RFC2328] for OSPF(with the value set as the 24-bit binary value of all ones:0xFFFFFF), MAX_PATH_METRIC in "IS-IS Extensions for Traffic Engineering" [RFC5305]and MAX_V6_PATH_METRIC in "Routing IPv6 with IS-IS" [RFC5308], with the value set as 0xFE000000(the 32-bit binary, 2^32 - 2^25)¶
It defines also two new flags U/UP, that are used to transfer the unreachable reason within the IGP domain, which does not fit into the role of IGP protocol.¶
The other parts of the document describe how to control the advertisement of unreachable information on ABR, its consideration with the area partition etc, which are first and throughly described clearly and more throughly in [I-D.wang-lsr-prefix-unreachable-annoucement](Founder Draft).¶
Even including these mimic parts, it still lacks the key consideration of the receiver behavior, which is important for the overall signaling effect.¶
After analyzing these issues, this document recommends the IETF community to abandon the forwarding of UPA draft, replace it with Founder Draft, which is one far earlier and more comprehensive solution.¶
[RFC2328] defines the LSInfinity feature, but doesn't give any reasonable explanation.¶
Actually, using the LSInfinity value for one prefix to indicate the prefix is unreachable is problematic in many scenarios in the network, even in the simple topology that illustrated below:¶
+----------------------------+----------------------------------------------------+ | +--+ +--+ +-+-+ ++-+ +--+ | | |R1+--(30)--+R2+---(30)--+ABR+--(20)--|R3+--(10)--+R4+--+-(LSInfinity-10)---P0 | | +--+ +--+ +-+-+ +--+ +--+ +-(LSInfinity)------P1 | | | | | Area 0 | Area 1 | +----------------------------+----------------------------------------------------+ Figure 1: LSInfinity defined in RFC2328 Is Flawed¶
In Figure 1, the value between the router is the cost of the link.¶
Suppose the Router R4 has two prefixes, one is P0, with the metric set to (LSInfinity-10); another is P1, with the metric set to LSInfinity.¶
R4 advertises these two prefixes within the Area1, with two different LSAs.¶
When R3 receives the LSA for prefix P0, according to the description in RFC 2328, it will treat the prefix P0 as reachable, because the cost within the LSA for prefix P0 is lower than LSInfinity.¶
When R3 receives the LSA for prefix P1, according to the description in RFC 2328, it will treat the prefix P1 as unreachable, because the cost within the LSA for prefix P1 is LSInfinity.¶
But on R3, the total cost to P0 and P1 are all LSInfinity, but one is reachable, another is unreachable, results in Fist contradiction conclusion.¶
When the above LSAs reach to ABR, ABR will take the similar treatment, that, it should treat prefix P0 as reachable, and prefix P1 as unreachable.¶
But according to the description in RFC 2328, "if the routing table cost equals or exceeds the value LSInfinity, a summary-LSA cannot be generated for this route", then ABR can't generate the summary LSA, neither for prefix P0(reachable, but total cost exceed LSInfinity), nor for prefix P1(Unreachable).¶
The routers within area0, R1 and R2 can't reach prefix P0 then. This is Second contradiction conclusion.¶
From the above examples, we can know the LSInfinity feature described in [RFC2328] is flawed. Such analysis can also apply to the MAX_PATH_METRIC in [RFC5305] and MAX_V6_PATH_METRIC in [RFC5308] used for the same purpose.¶
The IETF community should erase such flawed features, instead of exploiting it to signal unreachable information.¶
Then section 3 of UPA draft should be removed.¶
Section 5 of UPA draft defines two new flags: U and UP to signal the reason of unreachable information.¶
It says U flag is used to indicate "prefix is unreachable due to the unplanned reason.", the UP flag is used to indicate "the prefix is unreachable due to the planned reason".¶
These flags are not used to influence the SPF calculation of each router, just gives some ambiguous and insufficient reasons(for example, what's the specific unplanned reason?----node failure, link failure, configuration failure? And what's the plan of planned reason") for the unreachable, which is belong to the maintenance information, not required by IGP protocol.¶
Then such information should not be flooded within the IGP protocol.¶
Section 5 of UPA draft should be removed.¶
There are several parts in UPA draft mimic the descriptions in the corresponding parts of Founder Draft which described the related contents throughly about one year earlier.¶
Section 4 of UPA draft describes the generation of the UPA signalling, which mimic directly from Founder Draft. Even so, it lacks still other control knobs at the ABR, for example, how to balance the reachable and unreachable advertisement in some rigid conditions.¶
In order to maintain the code of conduct within IETF community, such mimic parts should be removed.¶
Section 6 of UPA draft describes the "deployment consideration for UPA", which is mainly for the duration of UPA signaling and area/domain partition, all these contents are first described in Founder Draft.¶
For area/domain partition, UPA draft can't give suitable solution, can only describe "UPA is not meant to address an area/domain partition.".¶
If so, section 6 of UPA draft should be removed.¶
Section 7 of UPA draft describes the process of the UPA should be controlled by the configuration at the receiver. But it lacks one key point, as described in Founder Draft, that the trigger action can only be activated when the receivers receives the UPA signal from all of the ABRs of one area. Or else, there will be misoperation.¶
The unique parts of UPA draft either should be discouraged(exploiting the flawed LSInfinity feature) or not appropriate(U/UP Flag, which floods maintenance information within IGP protocol).¶
The other parts of UPA mimic the Founder draft¶
Then, the IETF community should abandon the UPA draft, put forward to the Founder Draft instead.¶
The mechanism described in this document does not raise any new security issues for the IGP protocols.¶
TBD.¶
None.¶